1. Technical Field
The present invention relates to a cleaning method, and more particularly relates to a cleaning method which is capable of an ultrahigh level of cleaning using far fewer processes than conventional methods and without conducting heating operations.
2. Background Art
The semiconductor devices which are formed on semiconductor substrates have become more highly integrated and smaller, reaching the submicron level. In order to achieve this increase in integration, it is necessary to maintain the surface of the substrate in an ultraclean state. In other words, it is necessary to remove organic materials, metals, various types of particles and oxides (oxide films) from the surface of the substrate. It is for this reason that cleaning of the substrate is conducted.
In order to obtain a clean substrate surface, cleaning methods have been developed, all processes of which take place at room temperature, and in which chemical vapors are not generated.
In other words, a cleaning method is known (Japanese Patent Application, First Publication No. HEI 8-306655), which comprises a first process, in which cleaning is conducted by means of pure water containing ozone, a second process, in which, while applying vibrations having a frequency of 500 kHz or more, cleaning is conducted using a cleaning liquid containing HF, H2O2 and/or O3, H2O, and surfactant, a third process, in which cleaning is conducted by means of pure water, and a fourth process in which oxide films are removed.
In this method, the cleaning liquid containing HF, H2O2 and/or O3, H2O, and surfactant, which is employed in the second process, is a cleaning liquid which has very good cleaning properties, and the reactivity thereof is extremely high. This high degree of reactivity is extremely effective with respect to the substrate to be cleaned; however, because it is necessary to provide the members comprising the device with resistance to this highly active cleaning liquid, it is necessary to use materials for the device which have corrosion resistance. For this reason, a carbon layer is formed on top of a nickel fluoride layer in, for examples the inner surfaces of the cleaning liquid storage unit such as a metallic vessel (Japanese Patent Application, First Publication No. HEI 8-306655).
Furthermore, chemicals having good removal effects with respect to organic materials, metals, various types of particles, and oxides (oxide films) are employed as the solutions used in the cleaning of the semiconductor. However, in the cleaning process using pure water or ultrapure water which serves to conduct rinsing after cleaning with chemicals, there are no removal effects with respect to the organic materials, metals, various types of particles, or oxides (oxide films). On the contrary, there are reports of formation of oxide films as a result of the oxygen which is dissolved in the pure water or ultrapure water. Such growth of oxide films interferes with epitaxial growth after the oxide film removal process using, as an example, hydrofluoric acid. As a method for solving such problems, pure water or ultrapure water, termed deoxygenated water (deaerated water), in which the oxygen content in the pure water or ultrapure water is reduced to a level of a few ppb, is employed. Furthermore, it is known that the n+ silicon surface (indicating n type silicon in which the doping level is in excess of 1xc3x971019/cm3) experiences rapid oxidation. n+ silicon is extremely important as a contact material for the formation of metallic electrodes in silicon type elements. It is desirable that the n+ silicon surface remain unoxidized to the greatest extent possible, so that the contact resistance between the metal and the semiconductor does not increase. However, it is extremely difficult to suppress the growth of oxide films on n+ silicon surfaces simply by employing deaerated water.
In the wet cleaning process of the silicon substrate, the surface of the substrate has no oxide film thereon after cleaning using a cleaning liquid containing hydrofluoric acid. However, substrates in such a state are especially subject to the deposition of various types of particles. In particular, in cases in which there is only a rinse process using pure water or ultrapure water after cleaning using a cleaning liquid containing hydrofluoric acid, the pure water or ultrapure water themselves do not have the effect of removing particles, so that this leads to defects in crystallization arising from such particles in later processes, such as film formation processes and the like.
After cleaning the silicon substrate using chemicals containing hydrofluoric acid in the wet cleaning process, the silicon atoms at the outermost surface of the substrate bond with hydrogen atoms, so that the outermost surface of the substrate adopts a structure in which termination with hydrogen atoms is observed. This hydrogen-terminated silicon surface is known to be extremely chemically stable. However, since not all silicon atoms are bonded to hydrogen atoms, the presence of silicon atoms in an unbonded state at the surface and the presence of silicon atoms which are bonded with fluorine molecules, has been observed. Such silicon atoms are chemically extremely unstable and are likely oxidation sites.
The present invention has as an object thereof to provide a cleaning method, which is capable of room temperature processing without conducting heating, uses little chemicals and water, and does not require the use of special devices or materials.
The present invention also has as an object thereof to provide a cleaning method which, in the chemical cleaning process or the rinse process employing pure water or ultrapure water in the semiconductor wet cleaning process, restricts the formation of surface oxide films, removes, and prevents the deposition of, particles, and aids in the hydrogen termination of silicon atoms.
The cleaning method of the present invention comprises: a first process, in which cleaning is conducted using pure water containing ozone, a second process, in which, while applying vibration having a frequency within a range of 500 kHz-3 MHz, cleaning is conducted using a cleaning liquid containing HF, H2O, and surfactant, a third process, in which cleaning is conducted using pure water containing ozone, a fourth process, in which oxide films are removed, and a fifth process, in which cleaning is conducted using pure water.
In the cleaning method of the present invention, after cleaning the target material with chemicals, rinsing is conducted using pure water or ultrapure water containing hydrogen gas in an amount of 0.5 ppm or more, and containing dissolved oxygen gas in an amount of 100 ppb or less.
In the present invention, the first cleaning which is conducted employs pure water containing ozone (first process). In this first process, the majority of the metals and organic materials are removed. By conducting this first process, it is possible to minimize variations in the surface roughness after all cleaning processes.
After the first process, it is possible to begin the cleaning of the second process employing chemicals containing HF, H2O, and surfactant, without conducting ultrapure water cleaning. In other words, it is possible to omit one ultrapure water cleaning process ozone-containing ultrapure water remains on the substrate surface after the first process, but if the substrate proceeds to the second process with this water remaining on the surface thereof, there are no undesirable effects. It is preferable for the ozone concentration at this time to be within a range of 2 ppm-10 ppm. At amounts of less than 2 ppm, the removal of metals is insufficient, while when the amount is in excess of 10 ppm, the roughness of the substrate surface increases dramatically.
The second process involves cleaning using chemicals containing HF, H2O and surfactant, while applying vibration of 500 kHz or more; particles and metals present in oxide films are removed by this cleaning. Furthermore, it is also possible to reduce the surface roughness thereby. An anionic, cationic, or nonionic surfactant may be employed here. The use of a nonionic surfactant, which reduces the surface tension of the solution, is particularly preferable.
The HF concentration is preferably within a range of 0.05 weight percent to 0.5 weight percent. The HF concentration is more preferably within a range of 0.1 weight percent to 0.5 weight percent. At amounts of Less than 0.05 weight percent, the etching of the oxide films is too slow, and the removal of metals can not be conducted smoothly. When HF is present in an amount greater than 0.5 weight percent, the roughness of the substrate surface increases dramatically.
It is preferable that the frequency of the vibration employed be within a range of 500 kHz to 3 MHz. When the frequency is less than 500 kHz, a static charge is produced on the substrate as a result of the large amount of friction of the water droplets produced by the vibration, and this leads to gate breakdown in the device. Furthermore, when the frequency is in excess of 3 MHz, the efficiency of the amplifier worsens as the frequency increases, so that it is necessary to employ a large amount of power in order to achieve a high output, and this is not practical. Processing may be conducted in each process from process 1 to process 5 while applying vibration within a range of 500 kHz to 3 MHz.
In process 3, the surfactant employed in process 2 remains on the surface of the substrate, so that cleaning is conducted using pure water containing ozone, and the surfactant is completely removed. The cleaning process is simplified if the ozone concentration is set so as to be the same as that in the first process.
In process 4, since the surface of the substrate was oxidized by the cleaning using ultrapure water containing ozone of process 3, cleaning is conducted using a dilute HF solution, and the oxide film is removed. The concentration of the dilute HF solution employed is within a range of 0.05% to 0.5%.
In process 5, cleaning (rinsing) is conducted using ultrapure water, and the slight amount of chemicals which have been deposited on the surface of the substrate are removed. At this time, when hydrogen is added to the ultrapure water, the surface of the substrate can be cleaned without oxidation, and at the same time, it is possible to prevent redeposition of foreign matter. It is has been learned that sufficient effects are obtained when only a very small hydrogen concentration of 1.6 ppm or less is employed.
Additionally, a highly reactive cleaning liquid, such as that containing HF, H2O2 and/or O3, H2O, and surfactant is not employed, so that it is not necessary to form a carbon layer on a nickel fluoride layer in the inner walls of the cleaning liquid storage unit comprising a metal vessel, and the inner walls posses sufficient resistance to the cleaning liquid simply by the formation of a nickel fluoride layer on the inner walls of the metallic vessel comprising the cleaning liquid storage unit.
In the present invention, by first adding hydrogen gas to pure water or ultrapure water, the effect is achieved of suppressing the formation of an oxide film. It was learned that the solution was effective from the extremely low added hydrogen concentration of 0.5 ppm. Furthermore, it is preferable that the concentration of the oxygen also dissolved at this time be 100 ppb or less. When an amount of oxygen in excess of 100 ppb is dissolved, it is impossible to completely suppress the formation of oxide films. This phenomenon is particularly pronounced on the surface of n+ silicon.
By means of adding hydrogen gas to pure water or ultrapure water, it is possible to replace the lack of bonds, or other adsorbed atoms, on silicon with hydrogen atoms. By means of this, the hydrogen termination of the silicon surface is advanced, and since the silicon becomes less susceptible to electron exchange, the surface is stabilized. Effects were found from the extremely low added hydrogen concentration of 0.5 ppm. Furthermore, the concentration of the oxygen also dissolved at this time is preferably 100 ppb or less. When an amount of oxygen in excess of 100 ppb is present, the hydrogen-terminated surface is disrupted. This phenomenon is particularly pronounced at the surface of n+ silicon.
By adding hydrogen gas to pure water or ultrapure water, and applying a vibration having a frequency of 500 kHz or more, it is possible to remove particles and to prevent their redeposition. However, it is preferable that the frequency of the vibration employed be within a range of 500 kHz to 3 MHz. When the frequency is less than 500 kHz, a static charge builds up on the substrate as a result of the large friction of the water droplets produced by the vibration, and this leads to device breakdown, while when the frequency is in excess of 3 MHz, the efficiency of the amplifier worsens as the frequency increases, so that it is necessary to apply a large amount of power in order to obtain a large output, and this is not practical. Furthermore, the hydrogen concentration necessary for removal of the dissolved particles and for prevention of the redeposition thereof is extremely low, at 0.5 ppm. Furthermore, at this time, it is preferable that the concentration of the oxygen which is also dissolved be 100 ppb or less. When oxygen is present in an amount of more than 100 ppb, formation of an oxide film is observed on the silicon surface.
By means of adding ozone or hydrogen peroxide to the hydrofluoric acid, it is possible to add the effect of preventing redeposition of particles to the ability to remove silicon oxide films of the hydrofluoric acid. It is preferable that the hydrofluoric acid concentration be between 0.05 weight percent and 1 weight percent, that the ozone concentration be within a range of 2 ppm to 10 ppm, and that the hydrogen peroxide be present in an amount within a range of 0.1 weight percent to 1 weight percent. When the hydrofluoric acid concentration is less than 0.05 weight percent, the solution exhibits almost no ability to etch the oxide film of the silicon. Furthermore, when the hydrofluoric acid concentration is in excess of 1 weight percent, the roughness of the surface of the substrate becomes extreme. Furthermore, when ozone is present in an amount of less than 2 ppm or hydrogen peroxide is present in an amount of less than 0.1 weight percent, there is no effect of preventing particle deposition. Furthermore, when ozone is present in an amount greater than 10 ppm or hydrogen peroxide is present in an amount greater than 1 weight percent, oxide film remains on the surface of the substrate.
The pure water which is discussed here is water having a resistivity of 15 Mxcexa9/cm or more, and the ultrapure water is water having a resistivity of 18 Mxcexa9/cm or more.
The following effects and benefits are obtained by the present invention;
1) There are extremely few processes;
2) Chemical vapors are not produced;
3) Processing can be completed using little chemicals and water;
4) With respect to chemicals, only HF is employed, and this is not mixed with other chemicals, so that recovery is facilitated;
5) All processes can be conducted at room temperature without conducting heating; and
6) A mild cleaning liquid is employed, so that it is not necessary to provide the materials with strong resistance to the solution.
Further, it is possible to suppress the formation of a natural oxide film in pure water or ultrapure water.
It is also possible to chemically stabilize the silicon surface.
It is also possible to provide particle removal effects to the pure water or ultrapure water.
Further, it is possible to suppress the deposition of particles in the wet cleaning process.